1. Field of the Invention
The present invention relates to an image pickup apparatus and an image pickup system. More particularly, the present invention relates to a radiation image pickup apparatus and a radiation image pickup system applicable for use in taking a general still image or a moving image in fluoroscopy for use in medical diagnosis.
2. Description of the Related Art
In recent years, a radiation image pickup apparatus using a flat panel detector (hereinafter referred to as a FPD) produced using a semiconductor material has been used in practical applications such as a medical diagnosis based on an X-ray or radiation image, a nondestructive inspection, etc.
One of such image pickup apparatuses is a digital image pickup apparatus used to take a general still image or a fluoroscopic moving image for use in medical diagnosis. U.S. Pat. No. 6,115,451 and U.S. Pat. No. 6,920,198 disclose techniques of processing image data obtained by such an image pickup apparatus to produce an image with improved clearness. More specifically, for example, a correction process is performed on image data to reduce an offset or lag signal.
In U.S. Pat. No. 6,115,451, it is described that the quality of offset depends on current leakage of a detector, temperature, an amount of background radiation, and other such factors. In U.S. Pat. No. 6,920,198, it is described that the lag signal originates from an effect of an image signal generated in previous exposure to radiation, and such a lag signal can produce an artifact such as a ghost in an image. Hereinafter, the offset and the lag signal are generically referred to as “offset”, and a correction process to reduce the “offset” is referred to as an “offset correction”.
U.S. Pat. No. 6,115,451 discloses an offset correction technique in which a detector is first exposed to radiation and then the detector is read to acquire a value indicating an amount of radiation exposure. When a period greater than an exposure period has elapsed since the reading of the amount of radiation exposure, an offset value is read. The offset correction is then performed by subtracting the offset value from the value indicating the amount of radiation exposure.
U.S. Pat. No. 6,920,198 discloses an offset correction technique in which an offset prediction model is produced, and the offset prediction model is updated periodically at least once during a scanning period in which one fluoroscopic image of a subject is produced. To periodically update the offset prediction model, at least one dark image (offset image) is acquired. An offset correction of the fluoroscopic image is performed using the updated offset prediction model.
In U.S. Pat. No. 6,115,451 and U.S. Pat. No. 6,920,198, after image data based on values indicating amounts of exposure to radiation or a fluoroscopic image is obtained, correction image data (image data for use in correction) based on detected offset values or a dark image is obtained separately from the image data in a similar manner to the image data. The offset correction is performed using the obtained correction image data.
However, in U.S. Pat. No. 6,115,451, image data is acquired after exposure to radiation is performed, and correction image data is acquired when a period greater than an exposure period has further elapsed since then. Therefore, to obtain correction image data after exposure to radiation is started, it takes a time at least twice longer than a time needed to acquire image data after exposure to radiation is started. Thus, it takes an inconveniently long time to display offset-corrected image data after exposure to radiation is started.